In a typical loudspeaker system, the motor assembly includes a permanent magnet mounted between a top plate and a back plate, a pole piece centrally mounted on the back plate, and a voice coil axially movable with respect to the pole piece. During operation of the loudspeaker, electrical energy is supplied to the voice coil, causing the voice coil and attached diaphragm to move axially relative to the pole piece and within the air gap formed between the top plate and the pole piece. Heat produced by the voice coil can build up and be radiated to surrounding surfaces, particularly the top plate and pole piece. Eventually, this increasing voice coil temperature will lead to reduced power handling of the speaker and increased power compression.
The pole piece may be formed with a center vent which provides a flow path for the transfer of cooling air from outside of the speaker. Air flow through this vent is created in response to movement of the diaphragm with the excursion of the voice coil. However, such designs do little to directly cool the transducer voice coil, as air is simply pumped straight through the pole piece out the back of the motor. In fact, in some cases, a very large center vent can reduce convective cooling in proximity of the voice coil, and therefore reducing power handling of the loudspeaker system.
In some instances, holes or slots may be formed radially within the pole piece and extend outwardly from the center vent toward the voice coil in an attempt to provide convective cooling to the voice coil. Such radial holes may be effective to cause cooling air from the center vent to flow directly against at least a portion of the voice coil, but the position and shape of these holes or slots does not efficiently pull toward the voice coil and disturbs the laminar air flow within the center vent, creating turbulence and drag. Furthermore, an acoustic problem can be created with such radial slots, as a large amount of air is forced through a small passage.